Add automated test suite for the Flutter frontend

Introduce a headless `flutter test` suite (83 tests) covering models,
utilities, every API endpoint, and the five screens, with shared helpers
and fixtures so new tests stay terse.

API endpoint methods are statically-dispatched extensions on the
BackendAPI singleton, so they cannot be mocked via `implements`. Mock at
the Dio HTTP-adapter layer (http_mock_adapter) instead, enabled by two
small @visibleForTesting seams: BackendAPI.dioForTesting swaps the
singleton's Dio, and BackendReachability.forceOnlineForTesting() forces a
deterministic online state so polling widgets load.

Add frontend/run_tests.sh and document it in CLAUDE.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/test/unit/device_test.dart

@@ -0,0 +1,43 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/model/device.dart';
+import 'package:frontend/model/device_type.dart';
+
+import '../helpers/fixtures.dart';
+
+void main() {
+  group('Device.fromJson', () {
+    test('maps all snake_case fields', () {
+      final device = Device.fromJson(
+        deviceJson(
+          macAddress: '11:22:33:44:55:66',
+          ipv4Address: '10.0.0.5',
+          vendor: 'Globex',
+          lastSeen: '2026-05-30T08:15:00Z',
+          isRegistered: true,
+          owner: 'bob',
+          deviceType: 'network_appliance',
+          name: 'Router',
+        ),
+      );
+
+      expect(device.macAddress, '11:22:33:44:55:66');
+      expect(device.ipv4Address, '10.0.0.5');
+      expect(device.vendor, 'Globex');
+      expect(device.lastSeen, DateTime.parse('2026-05-30T08:15:00Z'));
+      expect(device.isRegistered, isTrue);
+      expect(device.owner, 'bob');
+      expect(device.deviceType, DeviceType.networkAppliance);
+      expect(device.name, 'Router');
+    });
+
+    test('leaves name null when absent', () {
+      final device = Device.fromJson(deviceJson());
+      expect(device.name, isNull);
+    });
+
+    test('falls back to unknown device type for unrecognised values', () {
+      final device = Device.fromJson(deviceJson(deviceType: 'spaceship'));
+      expect(device.deviceType, DeviceType.unknown);
+    });
+  });
+}